TWI571576B discloses a linear gear-driven power delivery mechanism which essentially comprises, sequentially, a power input rotating element, a ball ring, a power input clamping ring, a plurality of transmission spheres, a power output clamping ring, another ball ring and a power output rotating element. The transmission spheres are supported by a supportive rotating element and have their rotation controlled by a plurality of driving rods. The power input rotating element is resiliently connected to the power input clamping ring through a helical elastomer. The power output rotating element is resiliently connected to the power output clamping ring through another helical elastomer. The power input rotating element transmits power, by the ball ring, to the power input clamping ring such that the power input clamping ring and the power output clamping ring can clamp the transmission spheres, allowing the power output rotating element to output power through the other ball ring. However, the power input rotating element can stretch the helical elastomer, only if the power input rotating element rotates quickly enough, i.e., at or above a threshold rotation speed, to generate a centrifugal force great enough to permit the stretching of the helical elastomer; and, this is a prerequisite for clamping the transmission spheres by the power input clamping ring and the power output clamping ring and thereby outputting power from the power output rotating element. The conventional linear gear-driven power delivery mechanism is applicable to a bicycle, albeit inefficiently. The power input clamping ring and the power output clamping ring fail to clamp the transmission spheres whenever the power input rotating element rotates below the threshold rotation speed (for example, at the moment when the bicycle starts or when its two pedals are aligned vertically during a climb.) The failure to clamp the transmission spheres leads to idling, thereby rendering power transmission tardy and unsteady. The expedient solution to this problem is for the cyclist to cycle hard enough for the power input rotating element to reach the threshold rotation speed.
Therefore, it is imperative to provide a stepless transmission capable of operating continuously to enable instant, stable transmission of power.